1. Field of the Invention
The present invention relates to circuits for charging batteries and, more particularly, to load generated signals for maintaining batteries serially connected or for disconnecting the batteries from one another to permit charging each individual battery.
2. Description of Related Prior Art
Recently developed lithium polymer (LI-PO), lithium ion (LI-ION), and possibly other lithium based batteries (hereinafter collectively referred to as lithium battery) have a high power density which renders them preferable in applications where the size versus energy ratio must be as small as possible. As the chemistry of batteries has been changing, it is inevitable that the methodology for charging these batteries must change to render the charging procedure as effective and as safe as possible. Moreover, the charging circuits must be configured to minimize unacceptable heating of the batteries, afford relatively rapid charging and terminate charging when a full charge is achieved.
Lithium based batteries have become very popular and have been used in numerous applications in both consumer and commercial products because of their relatively small size, and light weight relative to their electrical energy capacity. Regrettably, numerous instances of these lithium batteries catching fire, or even exploding, have been reported. Typically, such fire/explosion has occurred due to an externally induced shorting of the battery resulting in discharge of a significant current and commensurate heating of the battery. Internal shorting due to poor battery manufacturing processes or lack of manufacturing controls has also been responsible for many of the fires/explosions. Other causes include poor charging and discharging methodologies.
While internal shorting of a lithium battery is essentially impossible to control by a user as it relates to design, configuration and manufacturing processes of the lithium battery, some degree of protection against it can be accomplished by proper selection of battery packaging material such as sheet metal, flame retardant material or material with high melting temperature. Further, packaging material with appropriate characteristic and strength can contain the effects of an exploding lithium battery.
Effects of external shorting of a lithium battery pack can also be contained with proper selection of packaging material as well as with usage of fuse, both resettable and non-resettable. However, the ideal approach to mitigating the effects of external shorting is to prevent it from happening.
During charging of a multi cell lithium battery a defective cell may cause high current flow and commensurate heating of one or more remaining cells with attendant risk of fire or explosion. This situation may even arise if one or more of the cells is at a voltage substantially less than the remaining cells. To avoid this problem, the chargers used are preferably connected to each individual cell and the charge rate (current) applied is a function of the voltage of the cell; quite often a charger can also monitor the voltage of each cell during the charging process. Thereby, the capacity and voltage of each of the cells can be maintained essentially uniform and avoid excessive heating, fire and explosion.